1. The Field of the Invention
The present invention provides a getter device advantageously shaped like a substrate for use in a thin film deposition system and a method for its use.
2. Background
Processes for depositing thin films onto substrates are widely used in the manufacture of a wide array of commercial products. Examples of these processes include the fabrication of integrated electronic circuits (ICs) in which circuits are formed on a semiconductor substrate; the manufacture of data storage media, such as compact disks (CDs), where a thin layer of aluminum is deposited onto a substrate of a transparent plastic; the production of computer hard disks where a magnetic material is deposited onto a substrate such as aluminum; and the production of flat panel displays in which active elements are commonly created on glass substrates. Processes for depositing thin layers are also being adapted to the developing field of micromechanical devices, where micron-scale mechanical structures are fabricated with similar techniques to those utilized in the production of ICs. The main industrial techniques for the deposition of thin layers include chemical deposition from the vapor phase and physical deposition from the vapor phase, widely known in the art as “chemical vapor deposition” and “physical vapor deposition” respectively or by their acronyms “CVD” and “PVD”.
In CVD processing, two or more gaseous species are caused to react in an evacuated chamber containing a substrate. The reaction product forms a solid deposit on the substrate in the form of a thin film or layer. The degree to which the chamber must be evacuated will vary according to the particular CVD process employed. Some systems, those known as low-pressure or alternately ultra-high vacuum, can require initial evacuations of the deposition chamber to a pressure value in the range of 10−8-10−9 mbar. Hereinafter, reference to CVD processing will refer to the low-pressure variants unless stated otherwise.
Physical vapor deposition (PVD) actually encompasses a number of different techniques that all share the following common features:                a target formed of a material to be deposited, generally having the shape of a squat cylinder or a disk, is positioned in a deposition chamber in front of a substrate and parallel thereto; and        the chamber is initially evacuated to a base pressure and thereafter back-filled with an inert gas, generally argon or another noble gas, to a pressure of about 10−2-10−5 mbar; a potential difference of some thousands volts applied between the supports of the substrate and the target (providing the latter with a cathodic potential) generates a plasma of electrons and positive ions in the space between the substrate and the target; the positive ions are accelerated by the electric field into the target causing atoms or “clusters” of atoms to erode or “sputter” off of the surface of the target and into the atmosphere of the chamber; material thus eroded condenses onto the substrate to form a thin film or layer.As is well known in the art, commercially useful processes frequently include the deposition of a plurality of successive thin layers which may be performed in a succession of deposition chambers or in a single chamber configured to perform multiple depositions. Hybrid processes, comprising aspects of both CVD and PVD processes are also well known in the art.        
It is further understood in the art that the properties of thin layer devices, particularly ICs, are strongly dependent on the presence of defects within the deposited layers. These defects are most commonly due to the inclusion of impurity atoms or molecules within the deposited layers. Consequently, it is important to minimize the possible sources of contamination in all processing steps. For example, contamination can be reduced by using components of the highest possible purity (reactive gases in the case of CVD, targets in case of PVD, and inert gases generally) and ensuring the highest cleanliness of all surfaces within the production system and especially within the gas distribution system and each deposition chamber.
Presently, to create high quality films and to do so with the greatest efficiency, thin film deposition processes are commonly performed in systems comprising a plurality of chambers, each configured for a specific operation. For example, deposition steps are performed in deposition chambers, while conditioning chambers can be configured for cleaning or thermal processing steps like pre-heating substrates. Systems comprising multiple chambers can be arranged linearly such that one is directly connected to the next. Alternatively, multiple chambers can be disposed around a central transfer chamber.
Chambers are connected to one another by means of valves that are normally opened only to allow the transfer of substrates from one chamber to another. Substrates are passed between chambers by automated substrate handling equipment, in general mechanical arms that are configured to grasp or support a substrate typically along an edge by the use of tangs, clamps, and guides. The valves and the automated handling equipment are typically configured to the dimensions of the substrates, and are thus designed to accommodate objects that are both thin and broad. Semiconductor substrates, for example, are generally circular, often with a machined flat segment or notch to indicate crystallographic orientation, with thicknesses between about 0.5 mm and about 1 mm and lateral dimensions between about 150 mm to about 300 mm. Substrates used in the production of flat panel displays, on the other hand, are commonly rectangular with thicknesses between about 1 mm and 5 mm and lateral dimensions between about 10 cm and 1 meter.
In order to guarantee the highest cleanliness possible, all chambers are generally kept under vacuum with the highest vacuum levels being maintained in the deposition chambers. As is well known in the art, higher vacuum levels are typically achieved through the use of a series of pumps, each intended to operate in a different pressure range. Evacuation is typically initiated with a low-vacuum mechanical pump (e.g. a rotary pump) that is effective down to a pressure range of about 1-10−2 mbar. Lower pressures can be achieved with medium and high-vacuum pumps such as turbomolecular or cryogenic pumps.
A simple example of a process system comprising multiple chambers arranged around a transfer chamber will serve to illustrate the pathway traveled by a substrate. Substrates are initially arranged in a suitably shaped carrier (e.g. a cassette or a pod) that is loaded into a first chamber. The inner walls of the carrier are provided with tangs or guides for the purpose of keeping the substrates separate from each other, and to simplify automated handling operations. A vacuum of about 10−5-10−6 mbar is achieved in the first chamber after the substrates are first introduced, and then a valve is opened between the first chamber and the transfer chamber. A mechanical arm removes a substrate from the carrier and transfers it to the transfer chamber where the pressure is maintained at a level lower than those in the first chamber, generally about 10−7 mbar.
Next, a mechanical arm is employed, for example, to transfer the substrate from the transfer chamber to a deposition chamber through a second valve. The mechanical arm places the substrate on a sample holder near the center of the chamber. Typically, the sample-holder is supported on a pedestal that is moveable in some systems. The ability to heat the deposition zone, the region within the chamber surrounding the sample holder, is generally provided in deposition chambers both to help degas the pedestal during the initial stages of evacuation and to promote more homogeneous depositions. Deposition chambers are frequently provided for this purpose with heating equipment, such as electrical resistors or infra-red lamps, to heat the deposition zone either from the inside or from the outside of the chamber through one or more quartz windows.
The vacuum level required for thin film depositions in most manufacturing processes is generally about 10−8 mbar, which requires between about 4 to about 12 hours to achieve. After a sufficient vacuum is obtained in a deposition chamber a thin layer can be deposited according to the technique for which the chamber is configured, for example CVD or PVD. The technique may also include one or more preliminary operations that need to be performed at the deposition vacuum level but before the actual deposition. After the deposition has been completed the substrate can then be transferred, again by means of automated handling equipment and transfer chambers, either to another chamber or back to a carrier to be removed from the system.
Ideally, systems for depositing thin films should always be kept isolated from the atmosphere. However, vacuum chambers must be opened periodically, for example, to perform maintenance on automated components, to clean interior surfaces, and in the case of PVD, to replace exhausted targets or to switch to a different target in order to use the chamber to deposit a different material. Each time a chamber is brought to ambient pressure its interior surfaces, the surfaces of equipment disposed within the chamber, and the surfaces of any targets will tend to adsorb atmospheric gases, in particular water vapor. These adsorbed gases are then continuously released into the chamber when the chamber is next evacuated. The balance between the release of adsorbed gases from interior surfaces, commonly known as degassing, and the gas removal (“pumping”) speed of associated vacuum pumps substantially determines the base pressure of the system, where the base pressure is the lowest pressure attainable in a commercially reasonable length of time.
Lowering the base pressure necessarily results in fewer impurities in the process atmosphere and is therefore desirable. Base pressures are typically between about 10−7 and about 10−9 mbar. At these vacuum levels the chamber cleanliness is generally considered acceptable for starting a new cycle of depositions. It will be appreciated, however, that in some instances it is desirable to perform depositions and other operations at a preset pressure above the level of the base pressure. A preset value above the base pressure may be used, for instance, during the cleaning of a target and in other preliminary operations. Additionally, it may be desirable to use a preset value above the base pressure for depositions where film quality is acceptable at such pressures. The further value inherent any time a preset value above the base pressure is used is that it can be achieved more quickly than the base pressure. Faster pump-down cycles lead to greater throughput and therefore to greater yields per unit time.
Typically, in order to improve a base pressure, each time a chamber is evacuated after being opened to the atmosphere it is simultaneously heated to a temperature in the range of about 100° C.-300° C. This treatment is commonly known as “baking.” During a bake the rate of degassing from the surfaces within a chamber is increased, thus removing much of the gas initially adsorbed from the atmosphere. It is well known that degassing can be further increased by increasing the pumping speed. Thus, increasing the pumping speed during baking further reduces the quantity of residual adsorbed gases on surfaces within the chamber. Put another way, more aggressive pumping during a baking operation results in a cleaner vacuum chamber.
All else being equal, providing a cleaner vacuum chamber while maintaining an equivalent pumping speed after baking will result in a lower base pressure in the chamber. This is so because lowering the amount of residual adsorbed gas on interior surfaces reduces subsequent degassing, and base pressure is a balance between degassing and pumping speed. It should also be noted that by providing a cleaner vacuum chamber while maintaining an equivalent pumping speed after baking one can more quickly achieve a specific pressure value above the base pressure. Therefore, in situations where further reducing the base pressure is not considered important, one can bring a vacuum chamber down to an operational pressure above the base pressure more quickly by improving the pumping during baking.
In the case of PVD, targets are subjected to an additional cleaning treatment after being exposed to the ambient atmosphere. This treatment, commonly known as “burnin,” comprises performing a deposition on a sacrificial substrate and requires between about half an hour and about 4 hours. The deposition thus performed erodes away the contaminated surface of the target to expose a fresh clean surface. The contaminated surface material is deposited on the sacrificial, or “dummy,” substrate which then may be discarded.
The use of getter materials and devices inside thin film deposition chambers has already been disclosed in patent application EP-A-693626, the publications of international patent application WO 96/13620 and WO 97/17542, and in U.S. Pat. No. 5,778,682. The European patent application EP-A-926258, in the name of the assignee of the present application, also discloses the use of getter systems in PVD processes.
Getter materials that have been used for the production of prior art getter devices include metals such as zirconium (Zr), titanium (Ti), niobium (Nb), tantalum (Ta), and vanadium (V), and the alloys of these metals. Additionally, these metals, alone or in combination, have been further combined with one or more other elements chosen from among chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), aluminum (Al), yttrium (Y), lanthanum (La), and the rare-earth elements, Commercially popular getter materials include the binary alloys Ti—V, Zr—V, Zr—Fe and Zr—Ni and ternary alloys Zr—Mn—Fe and Zr—V—Fe.
Getter materials have been formed into getter devices by various different methods according to the prior art. Getter materials may be formed through traditional metallurgical processes such as casting and working, but more commonly they are formed through powder metallurgy. Powder metallurgy allows the getter material to be prepared with a tailored particle size distribution and to be sintered into a body with good mechanical strength and a desired degree of porosity. For gettering purposes, porosity is advantageous to increase the specific surface area (surface area/gram of material) of the getter material in contact with the atmosphere to be pumped.
Where getter materials need to be applied in very thin layers other techniques are often employed. A getter material deposited on a support can be obtained, for example, by PVD. The preparation of getter devices by PVD is described in the publication of international patent application WO 97/49109. This technique provides the advantage of allowing the deposition of a getter material onto many kinds of supports, including glasses and ceramics. Advantageously, deposits obtained by PVD techniques tend not to be sources of particle contamination.
Additionally, getter materials can be deposited onto a support in the form of a powder. The deposition of powders can be carried out by cold rolling, as is well known in the field of powder metallurgy, however powders can only be applied to metallic supports. Another technique is to spray a suspension of getter particles in a suitable solvent onto a heated support, as described in patent application WO 95/23425 incorporated herein by reference. Furthermore the support may be coated with particles of a getter material according to an electrophoretic technique. In this technique, however, the support must be electrically conductive. Electrophoretic deposition techniques are described in U.S. Pat. No. 5,242,559 which is incorporated herein by reference. Finally, depositions of getter material powders onto supports can be carried out by a serygraphic technique, as described in the publication of international patent application WO 98/03987, also incorporated herein by reference. The serygraphic technique is particularly advantageous because it provides for depositing powdered getter materials onto widely different types of supports including metals and insulators. Further, the serygraphic technique allows for the formation of patterned deposits where a portion of a coated support surface, for example, remains uncoated.
The references described above that teach getter devices, however, disclose only systems that are fixed within a vacuum chamber and continue to remain in the chamber while depositions or other processes are performed. A significant disadvantage of the foregoing prior art is that substantial modifications are often required in order to configure a processing chamber to accommodate such getter devices and associated hardware such as heaters and shields. These modifications increase the size, the complexity, and the cost of a vacuum chamber, and increased size generally implies an increased internal volume and a longer evacuation time. Further, getter devices of the prior art are not easily removed from a chamber without opening it to the ambient atmosphere.
As is well known in the art, getter materials require for their operation an initial activation treatment at temperatures between about 250° C. and about 900° C. for times ranging between about a few minutes and about an hour. The specific time and temperature necessary to activate a getter will vary according to the particular composition of the getter material as well as other factors. It is further well known that activated getter materials cannot be exposed to partial pressures of reactive gases higher than about 10−3 mbar, and that unactivated getter materials cannot be activated at such pressures, because either act can result in the combustion of the getter material.
It will be appreciated that not all gases are reactive with respect to a getter material, and therefore total pressures above about 10−3 mbar can still be safe so long as the partial pressures of the reactive gases remains below about 10−3 mbar. Reactive gases typically include species such as oxygen, water, carbon dioxide, carbon monoxide, hydrogen, and in some instances nitrogen. Examples of gases that are non-reactive include the noble gases such as helium and argon. Therefore, it would still be safe to introduce a getter material into a vacuum chamber containing argon at a pressure of about 10−2 mbar so long as the sum of the partial pressures of any reactive gases remains below about 10−3 mbar. It would also be safe, under such conditions, to activate a getter material.
It is an object of the present invention, therefore, to provide a getter device that can be easily transported into and out of an evacuated chamber instead of being permanently installed. It is another object of the present invention to provide a method for the use of such a getter device for increasing the pumping of a vacuum chamber to achieve either a lower base pressure or to reach a set operating pressure more quickly.